The polymerization of 6-aminocapronitrile to form nylon polymer is disclosed in Greenewalt U.S. Pat. No. 2,245,129 and Curatolo et al. U.S. Pat. No. 4,568,736.
When 6-aminocapronitrile is produced by partial hydrogenation of adiponitrile, hexamethylenediamine and tetrahydroazepine, i.e., the latter compound represented by the formula: ##STR1## also coproduced. The hexamethylenediamine is easily removed from the mixture by simple distillation, but the tetrahydroazepine (hereinafter sometimes referred to as THA) is not easily separated. The presence of THA in the 6-aminocapronitrile (hereinafter sometimes referred to as 6-ACN) that is to be polymerized limits the molecular weight of the polymer and causes color and branching in the polymer. It is, therefore important that THA be removed from the 6-ACN before polymerization.
It is an object of the present invention to provide a simple and efficient method of obtaining 6-ACN that is free from THA.
It has now been found that by heating, 6-ACN to about 235 degrees C (approximately the atmospheric pressure boiling point of 6-ACN) the contained THA is slowly converted to compounds which can be separated from 6-ACN by simple distillation. Certain organic compounds can be added to the 6-ACN to reduce the time required to convert the THA.